There has been conventionally known a method for sending common synchronizing pulses to communication nodes of a communication network in which communication nodes are not connected in a loop form and for achieving operation synchronism. However, there are not yet communication networks having communication nodes connected in a loop form and adopting the above described synchronizing method.
On the other hand, in ATM communication systems in which a plurality of communication nodes sends and receives cells to and from each other, various switches such as space switches, time switches, and bus switches are used for cell switching.
As for configurations of ATM communication systems using bus switches, international standardization is being advanced. For example, there is a distributed queue dual bus (DQDB) method proposed in IEEE and described in "Distributed Queue Dual Bus Subnetwork of a Metropolitan Area Network", IEEE 802.6 (Dec. 6, 1990).
In this ATM communication system, a unidirectional bus is connected between one communication node and an adjacent communication node to transmit a cell. From the communication node which has received this cell, a unidirectional bus is connected to an adjacent communication node to transmit the cell. This ATM communication system is formed by thus connecting a plurality of communication nodes to their adjacent communication nodes with a unidirectional bus. Furthermore, this ATM communication system has a feature that it has a similar bus in the opposite direction as well. When there are, on the double buses, no cells transmitted by other communication nodes, each communication node multiplexes, on a bus, a cell inputted from an input line. In this way, each communication node transmits the cell. Furthermore, each communication node separates a cell addressed thereto from the bus. In this way, each communication node receives the cell. Data are thus transmitted and received between communication nodes at high speed.
In this ATM communication system, a cell synchronizing method described in "Introduction to B-ISDN", OHM SHA LTD., p. 120 is used as a synchronizing method for finding a cell head when demultiplexing cells.
In the ATM communication system using the distributed queue dual bus method, two buses having different communication directions form physical loops, respectively. In that case as well, logical start points and end points of logical buses are provided and traffic control is exercised in order to equalize bus use frequencies of communication nodes. That is to say, cells flowing on each bus are discarded at the end point of the bus. In other words, contents of the cells are not transmitted beyond the end point. Furthermore, each of the start point and the end point can be set in an arbitrary position on each bus and can also be altered. The logical start point of each bus, i.e., the start point of each loop is called a slot generator.
Each of two buses having different communication directions has a start point and an end point of. When a cell is to be transmitted from a certain communication node to another communication node, it is necessary to decide which of the two buses the cell to be transmitted should be multiplexed on. Conventionally, this selection of the transmission direction is made by providing a table in each communication node so as to allow the central processing unit to set a transmission direction in the table.
In this ATM communication system, cell demultiplexing processing is conducted in each node. Therefore, the cell is transmitted to an adjacent node after a delay of the processing time. In addition, a transmission delay to the adjacent node is also caused. That is to say, the phase of the head of the cell received by the adjacent node deviates from the phase of the cell in the transmitting node. In the receiving node, therefore, it is necessary to find the head of the cell having a deviated phase and conduct cell demultiplexing processing. In the receiving node, therefore, cell synchronizing is conducted by using a method of extracting a synchronizing signal from received data to find the head of the cell in each communication node as described in "Introduction to B-ISDN", OHM SHA LTD., p. 120.
That is to say, the conventional digital communication network using looped multiple buses can realize high-speed communication. For this realization, however, complicated devices and processing as heretofore described are needed.
(1) Between adjacent communication nodes, the head position of a packet is deviated by cell processing time. For bringing communication interface units into synchronism, therefore, a complicated synchronizing method is needed. In addition, if the position of a slot generator is altered, the phase of a cell after the position alteration of the slot generator varies from that before the alteration. This results in a drawback that every communication node temporarily gets out of synchronism.
(2) In order to determine the transmission direction of data, it is necessary to store relations between communication buses and transmission directions every communication node in the form of an exclusive table or in the form of integration with other tables and the central processing unit must update contents of this table and decide the transmission direction for each call. In case the position of a slot generator is altered, therefore, the transmission direction for every cell varies and hence tables disposed in respective communication nodes must be rewritten.
(3) In case communication nodes are duplicated in order to improve the reliability, two systems are recognized separately at the time of cell transmission and transmission is conducted. That is to say, the central processing unit must conduct transmission processing for each cell and for each system.
(4) In case information is to be transmitted from one communication node to a plurality of communication nodes among a plurality of communication nodes, i.e., when point-multipoint communication is to be conducted, a cell must be transmitted to both of the two buses. In the above described paper, however, its concrete method is not shown.